1. Field of the Invention
The present invention relates to an exhaust emission control device for an internal combustion engine, and in particular relates to an exhaust emission control device for an internal combustion engine for reducing and purifying nitrogen oxides contained in the exhaust gas.
2. Description of the Related Art
Conventionally, the air-fuel ratio of the mixture supplied to an internal combustion engine is set leaner than the stoichiometric air-fuel ratio, however when a so-called lean-burn control is performed there is a tendency for the discharge amount of nitrogen oxides (NOX) to increase. To address this there is known a technique for carrying out exhaust gas purification, by providing an exhaust emission control device incorporating a NOX absorbent for absorbing, or adsorbing, and removing NOX from the exhaust gas in the exhaust system of an internal combustion engine.
This NOX absorbent absorbs NOX in a condition where the air-fuel ratio of the exhaust gas is set to leaner than the stoichiometric air-fuel ratio so that the oxygen concentration in the exhaust gas is relatively high, and conversely reduces and purifies the absorbed NOX and expels it in the form of nitrogen gas (N2) in a condition where the air-fuel ratio is set to richer than the stoichiometric air-fuel ratio so that the oxygen concentration in the exhaust gas is low and there is an abundance of hydrocarbons and carbon monoxide and the like.
However, when NOX accumulates in the NOX absorbent the NOX absorption capacity thereof is reduced so that the purifying efficiency with respect to the exhaust gas is reduced. Therefore, it is necessary to desorb the NOX absorbed by the NOX absorbent at an appropriate timing. In this case there is known a technique for carrying out regeneration of the NOX absorbent by performing so-called rich-spiking which involves; temporarily setting the airfuel ratio of the exhaust gas flowing into the NOX absorbent richer than the stoichiometric air-fuel ratio to thereby desorb the NOX which has been absorbed in the NOX absorbent, and to reduce and purify the desorbed NOX by means of the hydrocarbons and carbon monoxide and the like contained in the rich atmosphere of the exhaust gas.
Here, it is known that in the case where the air-fuel ratio of the exhaust gas flowing into the NOX absorbent is set to richer than the stoichiometric air-fuel ratio, then while the NOX is being desorbed from the NOX absorbent, the air-fuel ratio of the exhaust gas discharged from the NOX absorbent becomes, if only slightly, leaner than the stoichiometric air-fuel ratio, and after this when the desorption of the NOX from the NOX absorbent is completed, the air-fuel ratio of the exhaust gas becomes richer than the stoichiometric air-fuel ratio. In this case, the smaller the amount of NOX absorbed in the NOX absorbent, the shorter the time taken after implementation of rich-spiking for the air-fuel ratio of the exhaust gas discharged from the NOX absorbent to change to richer than the stoichiometric air-fuel ratio . Therefore, for example as disclosed for example in PCT International Publication WO 94/17291, there is known an exhaust emission control device for an internal combustion engine which judges the degree of deterioration of the NOX absorbent, from the time taken after the implementation of rich-spiking for the exhaust gas discharged from the NOX absorbent to change to a rich atmosphere.
Furthermore, because sulfur is contained in fuel and lubricants for internal combustion engines, sulfur compounds such as sulfur oxides (SOx) are present in the exhaust gases. NOX absorbents such as those mentioned above also absorb sulfur compounds in addition to NOX. Here, the sulfur compounds absorbed by NOX absorbents have the propensity to accumulate more easily compared to NOX, and with an increase in the accumulated amount of sulfur compounds the absorption of NOX is hindered. Hence the purifying efficiency of NOX is gradually reduced and a deterioration of the NOX absorbent occurs due to so-called poisoning.
In the case where poisoning of the NOX absorbent occurs in this way, there is known a method for recovering such poisoning by removing sulfur compounds absorbed by the NOX absorbent by heating the NOX absorbent to an appropriate high temperature and setting the air-fuel ratio of the exhaust gas to richer than the stoichiometric air-fuel ratio and maintaining these condition for an appropriate period of time. As such an exhaust emission control device for an internal combustion engine, there is known an exhaust emission control device for an internal combustion engine as disclosed in Japan Unexamined Patent Application, First Publication No. Hei 7-186785, which performs the reduction and removal of sulfur compounds absorbed by the NOX absorbent when an estimated amount of sulfur compounds absorbed by the NOX absorbent exceeds a predetermined value, by controlling the temperature of the exhaust gas by controlling the transmission stage of an automatic transmission connected to the internal combustion engine, and setting the air-fuel ratio of the internal combustion engine to richer than the stoichiometric air-fuel ratio.
However, with the exhaust emission control device for an internal combustion engine according to the above-mentioned conventional technology, the amount of sulfur compounds absorbed by the NOX absorbents is estimated on the basis of the operating conditions of the internal combustion engine.
However, for the data considered necessary at the time of estimating the amount of these sulfur compounds, it is impossible to accurately know the amount of sulfur already contained in the fuel supplied to an internal combustion engine. Hence the problem arises in that the sulfur concentration within the NOX absorbent cannot be specified.
Moreover, even if the amount of sulfur compounds within the NOX absorbent are constant, since due to the temperature and the like of the NOX absorbent, the amount of sulfur compounds actually absorbed changes, there is a problem in that it is not possible to accurately judge the degree of deterioration of the NOX absorbent due to sulfur poisoning.
The present invention takes into consideration the above-mentioned situations with the object of providing an exhaust emission control device for an internal combustion engine which is able to accurately judge, by employing a simple method, whether the NOX absorption capacity of the NOX absorbent has deteriorated or not due to sulfur poisoning of the NOX absorbent.
In order to attain the object related to solving the above problem, the exhaust emission control device for an internal combustion engine according to the present invention which includes a nitrogen oxides purification device (the NOX absorbent 26 in the later described embodiment) provided in an exhaust system of an internal combustion engine for absorbing nitrogen oxides when an air-fuel ratio of exhaust gas is leaner than a stoichiometric air-fuel ratio, and reducing the nitrogen oxides when richer, comprising: an oxygen concentration measurement device (the second O2 sensor 24 in the later described embodiment) provided downstream from said nitrogen oxides purification device for measuring oxygen concentration in said exhaust gas; a time measurement device (the step S7 and step S8 in the later described embodiment) for measuring the time taken for an output signal from said oxygen concentration measurement device to change from lean to rich, when the air-fuel ratio of the mixture supplied to said internal combustion engine changes from a condition leaner than the stoichiometric air-fuel ratio to a richer condition; a threshold value setting device (the step S17 in the later described embodiment) for setting in relation to said time, a predetermined first threshold value (the sulfur poisoning stoichiometric conversion time threshold value tm: LMT-S deterioration, in the later described embodiment) for judging if a purifying capacity of said nitrogen oxides purification device has deteriorated due to sulfur poisoning; and a predetermined second threshold value (the heat deterioration stoichiometric conversion time threshold value tm: LMT heat deterioration, in the later described embodiment) shorter than said first threshold value for judging if said nitrogen oxides purification device has heat deteriorated; and a deterioration judging device (step S19 to step S22, in later described embodiment) for judging that said nitrogen oxides purification device has deteriorated due to sulfur poisoning when a measured time of said time measurement device is shorter than said first threshold value and longer than said second threshold value, and for judging that said nitrogen oxides purification device has heat deteriorated when the measured time is shorter than said second threshold value.
With the exhaust emission control device for an internal combustion engine of the above construction, in relation to the time (stoichiometric conversion time) taken for an output signal from the oxygen concentration measurement device to change from lean to rich when the air-fuel ratio of the mixture supplied to the internal combustion engine changes from a condition leaner than the stoichiometric air-fuel ratio to a richer condition, the measurement of the stoichiometric conversion time is carried out with respect to for example the nitrogen oxides purification device already poisoned by sulfur and the nitrogen oxides purification device which has heat deteriorated. Then based on these data, by setting the first threshold value for judging if due to sulfur poisoning the nitrogen oxides purification device has deteriorated to the extent that it is recoverable, and the second threshold value for judging if due to heat deterioration and the like the nitrogen oxides purification device has been irreversibly heat deteriorated to the extent that the NOX absorption capacity has been lost, it is possible to accurately distinguish between whether a nitrogen oxides purification device has heat deteriorated to an extent that the NOX absorption capacity has been lost and is thus unrecoverable, or whether it is in a temporary sulfur-deteriorated condition which is recoverable.
Furthermore, an exhaust emission control device for an internal combustion engine according to a second aspect provided a sulfur deterioration regeneration device (step S23, in the later described embodiment) for recovering sulfur poisoning of said nitrogen oxides purification device, when judged by said deterioration judging device that said nitrogen oxides purification device has deteriorated due to sulfur poisoning.
With the exhaust emission control device for an internal combustion engine of the above construction, it is possible to reliably judge if the nitrogen oxides purification device is in a temporary sulfur-deteriorated condition which is recoverable. Therefore, it is possible, with appropriate timing, to recover sulfur poisoning by means of the sulfur deterioration regeneration device, before deterioration of the nitrogen oxides purification device due to sulfur poisoning progresses. In addition, a lowering in purifying efficiency with respect to the exhaust gas can be suppressed. Furthermore, because implementation of anything over and above what is needed for implementing the processing for recovering the sulfur poisoning is prevented, it is possible to contribute to the preventing a worsening in the fuel consumption of the internal combustion engine.